Comparison of discharge estimates from a rating curve and ADCP measurements

Themes: (T) Special session, (ST) Acoustic monitoring of flow, turbulence and river discharge

Parallel session: 10F. Special session: Acoustic monitoring of flow, turbulence and river discharge

River discharge cannot be directly measured and is usually inferred from the stage by means of a rating curve. An unambiguous relationship between the stage and discharge is required for this method to be reliable. Due to backwater effects and tides no unique stage-discharge relationship may exist. In such cases the velocity has to be measured in addition to the stage to estimate the discharge. As most permanently deployment instruments measure the flow velocity only in a small fraction of the entire cross section, the discharge has to be estimated using a model such as the index velocity method. These models rely on a predictable relation between the velocity in the measured part and velocity averaged over the entire cross-section. In contrast to the water surface which is nearly constant over the river width, the flow velocity can vary considerably within a cross section. In addition to large-scale turbulent motions and bed form development, systematic variation of the velocity distribution is caused by side walls effects, irregular shape of the cross-section and channel curvature. These aspects may render velocity based discharge estimates less accurate than rating curve estimates. In this paper the accuracy of discharge estimates obtained by a rating curve and H-ADCP data processed with the index velocity method are compared, using field data collected in the Kapuas river, West Kalimantan, Indonesia. It is shown that the rating curve is more accurate for this particular deployment. A method is given to establish the necessary conditions for an HADCP deployment to achieve a certain accuracy.